[GDC] Evaluation order: Please update the dmd backend

[Iain Buclaw]

On 2 Apr 2014 09:52, "Sarath Kodali" <sarath at dummy.com> wrote:
>
> On Tuesday, 1 April 2014 at 22:04:43 UTC, Timon Gehr wrote:
>>
>> On 04/01/2014 08:40 PM, Sarath Kodali wrote:
>>>
>>> ...
>>>
>>> The evaluation order of assign operators should not be LTR as they have
>>> right associativity. In "a = b = c", c has to be evaluated first, then b
>>> and then a. Similarly, in "a = b + c", "b+c" has to be evaluated first
>>> before a is evaluated. Otherwise it will be very confusing, that in some
>>> cases it is LTR and in some it is RTL.
>>
>>
>> Note that this is after a paragraph that suggests to make evaluation in
some cases LTR and in some RTL.
>>
>
> There are 2 evaluation orders that need to be considered while evaluating
expressions - the evaluation order of operators and the the evaluation
order of operands of an operator. The evaluation order of operators is well
defined and is done according to its precedence and associativity. However
the evaluation order of operands for some of the binary operators is not
defined. D left it undefined for assign operator. So in "a=b", the compiler
can choose to evaluate a first and then b. However in "a=b=c", "b=c" has to
be evaluated first due to right associativity of '=' operator. Similarly in
"a=b+c", "b+c" has to be evaluated first due to higher precedence of +
operator over = operator.  In both these cases, the right operand of =
operator is evaluated first and then the left operand. So it naturally
follows that even in the unspecified case (a=b), the right operand should
be evaluated first so that it is consistent with other cases of = operator.
All this means, the evaluation order of operands also should be according
to the associativity of its operator. You can test this with other right or
left associative binary operators.
>
>
>
>>> Other binary operators like "+" have left associativity, and hence
>>> evaluation for these should be LTR as mentioned in D spec.
>>> ...
>>
>>
>> What's the presumed relation between associativity and evaluation order?
>>
>> In particular, the ternary operator ?: is right associative. How on
earth are you going to evaluate it right to left?
>>
>>> The C spec requires that the function arguments are to be pushed in RTL
>>> order.
>>
>>
>> [citation needed]
>>
>
> You can get that info from any C ABI doc from Intel or AMD or some other
arch.
>

That's order of pushing arguments, not order of evaluation.  Also, heavy
stress on the words *Intel* and *AMD*.  That is in no way a C standard. :)
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.puremagic.com/pipermail/digitalmars-d/attachments/20140402/e9be1e89/attachment.html>